@gschenk I did what you suggested, and since then it stopped creaking. Let's hope it doesn't come back. Thanks! But I also noticed some flaking at the edge of the brake pads (after turning the bike upside down—no work stand here). It could have been that. I removed the part that was flaking off.
I borrowed a chain wear tool. It can show "fine", 0.75 or 1.0 (and nothing inbetween those). It either shows 0.75 or "fine", depending on where exactly I measure. I guess that's still fine for a while? It's a 9-speed.
